To transmit time and frequency standards with accuracies approaching one part in 10.sup.14, feedback systems have been conventionally utilized to stabilize the transmission path against external influences. Some existing systems utilize coaxial lines as the transmitting path. For stabilization, a signal at the receiving end of the path is transmitted through the same path back to the transmitting end where its phase is compared with a standard reference source. Any phase fluctuations at the end of the path are corrected by a voltage-controlled phase shifter inserted in the path. However, reflections along the coaxial line utilizing this method are highly undesirable as they add signals travelling in opposite directions and vary the phase of the originally transmitted signal. Attempts to avoid this problem have resulted in utilizing forward and backward signals of different frequencies so that they can be easily isolated with filters. However, line dispersion and dispersion in the voltage-controlled phase shifter produce errors in phase correction which render it impossible to achieve an accuracy of one part in 10.sup.14. The apparatus and method provided by the invention eliminate these problems by transmitting the reference signal as modulation upon an optical signal, thus allowing a fiber optic transmission line to be utilized as the transmission path. The apparatus includes a novel optical phase shifter that is utilized to stabilize the propagation delay through the fiber optic transmission line.